1. Field of the Invention
This invention relates to a control system for a fluid coupling which comprises an input element, an output element and a lockup clutch and is connected to a transmission mechanism mounted on a vehicle, and more particularly to a control system which controls the slippage of the lockup clutch relative to the input element or the output element.
2. Description of the Prior Art
As a torque converter, a fluid coupling which associates with a transmission mechanism to form an automatic transmission for a vehicle, one having a lockup clutch in addition to a pump impeller as the input element and a turbine runner as the output element has been known. In the vehicle provided with an automatic transmission comprising a torque converter with a lockup clutch, there has been proposed to cause the lockup clutch to engage in such a way as to allow some slippage therein thereby producing a speed difference between the pump impeller and the turbine runner when the vehicle is running under a predetermined condition in order to minimize energy loss in the torque converter which occurs when the automatic transmission transmits torque to the wheel driving system from the engine and to suppress vibration of the vehicle body which is generated when fluctuation in torque is transmitted to the wheel driving system from the engine. The engagement of the lockup clutch which produces a speed difference between the pump impeller and the turbine runner will be referred to as "slipping engagement", hereinbelow, and the control which causes the lockup clutch to engage in slipping engagement will be referred to as "slip control" in this specification.
Usually such a slip control of the lockup clutch in the torque converter is effected under a predetermined running condition of the vehicle defined by the engine load and the vehicle speed. For example, as disclosed in Japanese Unexamined Patent Publication No. 57(1982)-33253, the slippage in the lockup clutch is controlled in a feedback control by the use of a controlled variable which is determined on the basis of the difference between a target speed difference between the pump impeller and the turbine runner of the torque converter and the actual speed difference between the pump impeller and the turbine runner. Further as disclosed, for instance, in Japanese Unexamined Patent Publication No. 60(1985)-14653, it has also been proposed to bring the lockup clutch into a particular state of operation free from the controlled variable for a predetermined time interval when the transmission mechanism shifts while the lockup clutch is in the slipping engagement, thereby suppressing shock accompanying the gear shifting.
In accordance with the proposal disclosed in Japanese Unexamined Patent Publication No. 60(1985)-14653, the shock accompanying the gear-shifting can be suppressed to some extent, but since the operation of the automatic transmission differs according to whether the gear-shifting is upshift or downshift, the shock will be better suppressed if the torque converter, more particularly the lockup clutch, is controlled in different ways according to whether the gear-shifting is upshift or downshift.